A cathode ray tube is an electron tube in which a beam of electrons is focused to a small area and varied in position and intensity on a surface. The surface referred to is cathodoluminescent, that is, luminescent under electron bombardment. In such tubes, the output information is presented in the form of a pattern of light which can be perceived by the eye. The character of the pattern is related to, and controlled by, one or more electrical signals applied to the cathode ray tube as input information.
The most familiar form of the cathode ray tube is the television picture tube, found in home television receivers. Cathode ray tubes are also used in measuring instruments such as oscilloscopes, which have been indispensable in laboratories devoted to experimental studies in science and engineering. For navigation, the cathode ray tube is the output device of radars. Increasingly, cathode ray tubes are finding use in input/output terminals of digital computers. Cathode ray tubes can display information quickly, using formats that are much less restrictive than other output devices such as printers. Cathode ray tubes are put to numerous other uses in science, industry and the arts.
The basic elements of a cathode ray tube are the envelope, electron gun and phosphor screen. While all of these elements are well known in the art, and do not need to be described in detail, a brief description of the electron gun is helpful in understanding the context of the present invention.
The electron gun produces, controls, focuses and deflects an electron beam. The electron gun consists of an electrical element called a heater, a thermionic cathode, and an assemblage of cylinders, caps and apertures which are all held in the proper orientation by devices such as glass beads, ceramic rods and spacers. The thermionic cathode is the source of the electrons in the electron beam. The emitting material is generally a barium-calcium-strontium oxide coating deposited on the end of a deep-drawn nickel cup with cylindrical walls. The oxide coating emits electrons when heated. The walls of the cup enclose a coiled tungsten wire coated with a refractory insulating material such as aluminum oxide. The passage of current through this wire generates sufficient heat, transmitted by conduction and radiation to the cup, to maintain the oxide coating at emitting temperatures on the order of 1100.degree. K. The heater-cathode structure is supported coaxially within, and insulated from, a control grid. The control grid along with other elements of the electron gun controls the intensity and direction of the beam.
A common technique for fabricating thermionic cathodes is to deep-draw the cathode sleeve from a bimetallic laminate. The resulting cathode sleeve comprises a laminated bimetallic member having a first layer and a second layer. The laminated bimetal layers are shown in FIG. 1, which illustrates a prior art cathode sleeve formed by a deep-drawing process. The first, or inner, layer typically comprises Nichrome.RTM.. The second, or outer, layer typically comprises electronic grade nickel. The deep-drawn cathode sleeve is then selectively etched in a mixture of acids to remove the outer layer of nickel from all portions of the cathode sleeve except at the closed end, as shown in FIG. 2. The etching process restricts the nickel layer to an end cap, and exposes the Nichrome.RTM. layer which has a lower thermal conductivity than the nickel layer. As those skilled in the art will understand, it is desirable to lower the thermal conductivity of the cathode sleeve to concentrate the heat at the closed end. This minimizes heat losses which increase the time required for heating up the cathode material to the operating temperature through the heat energy supplied by the heater. It is, of course, possible to minimize heating time by increasing the amount of current supplied to the heater, but this increases power consumption and is generally considered to be undesirable. In addition, minimizing heat losses also maintains the transmission efficiency of heat from the heater to the cathode, to obtain a desired thermal electron emission from the cathode. Removing the nickel layer from all portions of the cathode except the end cap has been a typical solution to the problem.
The etching (nickel removal) process also causes the undesired result of increasing the emissivity of the cathode sleeve's outer surface. Ideally, the outer surface should have low emissivity because the lower the thermal emissivity on the outer surface, the higher the cathode thermal efficiency. In general, smooth surfaces will exhibit lower emissivity than rough or irregular surfaces. Prior to an etching or surface treatment process, a typical bimetal laminate of nickel and Nichrome.RTM. will have a smooth nickel outer surface. However, after an etching process removes the nickel, the resultant surface will consist of an irregular or rough surface of Nichrome.RTM.. This irregular surface will have a high emissivity. Smoothing the irregular surface, if even possible at all, would require additional manufacturing steps.
Clearly, etching with acids presents numerous disadvantages. Moreover, the etching process must be precisely controlled, since typically the cathode parts are quite small (on the order of less than 0.350 inch in length and only about 0.075 inch in diameter, and on the order of 0.025 grams on mass). Small variations in the etching process can produce unacceptably wide variations in the finished parts, or even render the finished parts useless.
The present invention eliminates the need for acid etching and other finishing steps subsequent to deep-drawing without sacrificing the desired thermal characteristics of the cathode.
The prior art discloses that internal blackening or oxidizing of a thermionic cathode enhances certain operating characteristics of the cathode. In particular, such blackening or oxidizing creates a high heat radiating surface, and thereby increases surface emissivity. Surfaces with low emissivity are good reflectors of thermal energy, whereas surfaces with high emissivity are good absorbers of thermal energy. Thus, for a given energy input, a blackened cathode will reach a higher temperature than a non-blackened cathode (due to greater absorption of thermal energy), and thereby will have a higher thermal efficiency. The oxide layer on the inside surface of a blackened cathode, if made thick enough, will also improve the dielectric strength of the heater-cathode interface.
Typical bimetallic laminates of nickel and Nichrome.RTM. are blackened or oxidized by simultaneously heating and exposing the laminate to a wet gas environment. In this process, the chromium in the Nichrome.RTM. reacts with oxygen in the water vapor and forms chromium oxide. Since the nickel layer does not contain any oxygen reacting compounds, it is unaffected by this environment and does not undergo any changes in property. For example, in U.S. Pat. No. 3,958,146, a formed cathode cap or top cap is fired for about 10 minutes or longer in a wet dissociated ammonia at a temperature of about 900.degree. Celsius to 1,300.degree. Celsius to oxidize the available chromium on the surface of the Nichrome.RTM.. In U.S. Pat. No. 4,370,588, a cathode sleeve is heated at temperatures of 1,000.degree. Celsius for 30 minutes in a hydrogen environment containing water at a dew point of 20.degree. Celsius, thereby covering the surface of the cathode sleeve with chromium oxide. U.S. Pat. No. 4,554,480 discloses oxidizing (blackening) the inner and outer surfaces of an eyelet of a cathode assembly. However, in this patent, the eyelet is made of either 52 Alloy (a nickel/iron composition) or type 304 stainless steel, instead of Nichrome.RTM.. If the 52 Alloy is employed, it is oxidized by firing the eyelets at about 800.degree. Celsius for about 10 minutes in a wet nitrogen (N.sub.2) atmosphere. If the type 304 stainless steel is employed, it is oxidized by firing at a temperature of about 1000.degree. Celsius for about 10 minutes in a wet hydrogen atmosphere.
One major disadvantage associated with blackening thermionic cathodes in this manner is that both inner and outer surfaces of the Nichrome.RTM. layer (or eyelet in the case of U.S. Pat. No. 4,554,480) become blackened. Ideally, only the inner surface should become blackened. Blackening the outer surface causes at least two problems. One problem is that thermal energy emitted from the outer surface side walls causes a radiation cooling effect, as well as causing an increase in the temperature of the surrounding environment. This latter problem increases the probability of inter-electrode arcing. The goal of the designer is to concentrate heat at the closed end of the cathode so as to maximize thermionic emission. Blackening the outer surface enhances undesired thermal energy emission from the side walls.
Another problem with blackening the outer surface is that the oxide layer makes it difficult to weld other parts to the cathode structure. Thus, when an oxide layer is formed by simultaneously exposing inner and outer surfaces to a wet gas environment, only a very light (i.e., thin) layer can be allowed to form so as to ensure that parts can still be welded together on the outer surface. Accordingly, the light layer on the inner surface does not allow one to take full advantage of the benefits of inner surface blackening. In the prior art, there is no simple way to ensure that only the inner surface is exposed to the wet gas environment.
Turning again to prior art FIG. 2, it should be evident that subjecting cathode sheath 100 to a heated wet gas environment will cause blackening of the entire inner surface of the Nichrome.RTM. layer (a desired result) but will also cause blackening of the outer surface portion of the Nichrome.RTM. layer exposed by the etched away nickel layer (an undesired result).
The present invention allows for the construction of a thermionic cathode having a blackened inner surface and a smooth unblackened outer surface. Furthermore, a cathode sheath made in accordance with this invention will not suffer from the drawbacks described above.